Acetylenes trans-alkenes

LiAlH4 (lithium aluminium hydride) Ethers -78 to RT ester —> alcohol ketone —> alcohol aldehyde —> alcohol alkyl halide -> alkane acetylene —> trans alkene epoxide — alcohol imine - amine amide - amine... 

Acetylenic precursors employed in the syntheses of sugars may be divided into three groups (a) aldehydes (usually in the form of acetals), (b) alkyl alkynyl ethers, and (c) alkynols or alkynediols. Some of them are commercially available (for example, 2-butyne-l,4-diol), and others are prepared by Grignard-type reactions between 1-alkynylmag-nesium halides or lithium alkynes and suitable aldehydes, ketones, or epoxides. In this way, the synthesis of substrates having the desired number of carbon atoms, as well as the necessary functional groups, can be achieved. The next step consists in partial saturation of the triple bond to afford the desired cis- or trans-alkene. ct.s-Alkene systems... 

The electron transfer to the acetylenic bond forms the frans-sodiovinyl radical 20 that, after protonation, produces tram radical 21. At low temperature (—33°C) in the presence of excess sodium, the conversion of the trans radical to sodiovinyl intermediate 22 is slightly more rapid than the conversion of the tram radical to the cis radical 23 (21 —> 22 > 22 —> 23). As a result, protonation yields predominantly the trans alkene. However, low sodium concentration and increased temperature lead to increasing proportion of the cis alkene. Although other dissolving-metal reductions are less thoroughly studied, a similar mechanism is believed to be operative.34 Another synthetically useful method for conversion of alkynes to trans alkenes in excellent yields is the reduction with CrS04 in aqueous dimethylforma-mide.198... 

Open-chain acetylenes yield pure trans alkenes on reduction with alkali metal in liquid ammonia. However, reduction of cyclodecyne (111) with sodium in liquid ammonia led to a mixture of cis- (112, > 90%) and /rn/w-cyclodecene (113, > 4%y -The formation of the cis isomer (112) was attributed to the reduction of 1,2-cyclo-decadiene (114) formed by a rapid isomerization of the starting cycloalkyne, while the formation of the trans isomer (113) was ascribed to the direct reduction of 111. 

An important feature of the LH form of the hydride is that it contains a Co111 bearing a very open coordination site for reaction with nucleophiles. There are several publications on the unusual reaction of naked cobalt111 in porphyrins with acetylenes and olefins.25611 J-258 This could also explain the trans addition of the hydrides to acetylenes. A charge-transfer intermediate in the reaction of the LH isomer with an acetylene or alkene could explain the difference in behavior of substituted versus unsubstituted olefins. If the above explanation is correct, then it would require essential equality between the energies of the Co—H and L—H complexes. Parameters such as solvent properties could shift the equilibrium concentrations of the two isomers of the hydrides, leading to apparently different results in the same reaction and explain poor reproducibility in stereoselectivities.232,233... 

Otherwise few of these acetylene adducts are important in their own right, but they are valuable intermediates because disubstituted acetylenes (8) can be reduced to cis or trans alkenes at will and because monosubstituted acetylenes can be hydrated to methyl ketones (9). 

Conversion of an acetylenic compound to a trans alkene is achieved with sodium or lithium and liquid ammonia though this reduction is not usually as simple as that using Lindlar s catalyst. Alternatively the cw-alkenes can be converted to raw5-alkenes by stereomutation (Section 10.9) and the trans isomer isolated from the equilibrium mixture by crystallization and/or silver ion chromatography. 

The hydrogenation of acetylenes catalyzed by the dinuclear compound HRh[P(0-i-C3H7)3]2 2 is an interesting example of stereo selective cluster catalysis. At 20 °C acetylenes are transformed into trans-alkenes with a rate of 1 turnover per minute in a totally stereo-selective reaction. The mechanism proposed for this process is illustrated in the scheme in Fig. 2.73. This kind of... 

The solution of ytterbium in liquid ammonia appears to be an effective reducing agent [87]. It reduces aromatic compounds to 1,4-dihydroaromatic, unsaturated ketones to unsaturated derivatives and acetylenes to trans-alkenes. 

Reduction of acetylenes with sodium in ammonia, lithium in low molecular weight amines or sodium in hexamethylphosphoric triamide containing r-butanol as a proton source all lead to predominantly the trans-alkene. This method supplements LiAlH4 reduction as a means of converting acetylenes to trans-alkenes. The reaction is assumed to involve successive electron transfer and proton transfer steps. 

The addition of (TMS)3SiH to a number of monosubstituted acetylenes has also been studied in some detail. These reactions are highly regioselective (anti-Markovnikov) and give terminal (TMSlsSi-substituted alkenes in good yields. High cis or trans stereoselectivity is also observed, depending on the nature of the substituents at the acetylenic moiety. For example, the reaction of the alkynes 23 and 24 with (TMSlsSiH, initiated either by EtsB at room temperature (method or by thermal decomposition of di-ferf-butyl peroxide at 160 °C... 

Triple bonds in side chains of aromatics can be reduced to double bonds or completely saturated. The outcome of such reductions depends on the structure of the acetylene and on the method of reduction. If the triple bond is not conjugated with the benzene ring it can be handled in the same way as in aliphatic acetylenes. In addition, electrochemical reduction in a solution of lithium chloride in methylamine has been used for partial reduction to alkenes trans isomers, where applicable) in 40-51% yields (with 2,5-dihydroaromatic alkenes as by-products) [379]. Aromatic acetylenes with triple bonds conjugated with benzene rings can be hydrogenated over Raney nickel to cis olefins [356], or to alkyl aromatics over rhenium sulfide catalyst [54]. Electroreduction in methylamine containing lithium chloride gives 80% yields of alkyl aromatics [379]. 

The trans acetate 25 is the pheromone used to trap pea moths.6 Changing the -alkene into the acetylene allows the disconnection next to what was the double bond. The only problem is how to make a mono-bromide from the symmetrical diol 30. 

Muscalure, the sex attractant of the common housefly, is cA-tricos-9-ene. Most syntheses of alkenes give the more stable trans isomer as the major product. Devise a synthesis of muscalure from acetylene and other compounds of your choice. Your synthesis must give specifically the cis isomer of muscalure. 

Cycloadditions in which 1,2-dithietes acted formally as dienes are among the most typical reactions of 1,2-dithietes. The dithiete 144 is highly reactive and capable of reactions even with simple alkenes and alkynes (60JA1515 61JA3434,61JA3438). Thus, 144 reacted with acetylene to form 191 and 192 with the initial formation of 193, and with tetramethylethylene to give 194. Other [4 + 2] cycloadditions of 144 involved those with ethylene, cyclohexene, trans-stilbene, ethyl vinyl ether, butyl vinyl sulfide, 3-hexyne, and DMAD. 

Lithium acetylide is best prepared from acetylene and LiNH in liquid ammonia. Lithium acetylides are more soluble in liquid ammonia than sodium acetylides and therefore give higher yields (50-80%) on reaction with higher halides. trans-2-Alken-4-ynols have been generally obtained on alkylation of sodium acetylides with... 

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